1. Technical Field
The present invention relates to a charged-particle beam apparatus employing a charged particle beam such as an electron beam or ion beam. In particular, the invention relates to a charged particle beam apparatus suitable for obtaining high-resolution images with a minimum loss of resolution in cases where the charged particle beam is inclined with respect to a sample.
2. Background Art
In charged particle beam apparatuses such as those represented by the scanning electron microscope, a sample is scanned with a narrowly focused beam of charged particles in order to obtain desired information from the sample (such as a sample image). In such charged particle beam apparatuses, higher resolution is continuously becoming possible in recent years, and there is a need to obtain inclined images of a sample by inclining the charged particle beam with respect to the sample. An inclined sample image can be generally obtained by tilting the sample stage. However, it is more practical to tilt the charged particle beam relative to the sample than to mechanically tilt the sample stage, from the viewpoint of preventing a field of view error at higher magnification ratios, or increasing the rate at which inclined sample images are obtained. Hence the need to tilt the charged particle beam.
JP Utility Model Publication (Kokai) No. 55-48610 U (1980) and JP Patent Publication (Kokai) No. 2-33843 A (1990) disclose techniques for inclining a charged particle beam while maintaining the high resolution condition of the system. These techniques cause a charged particle beam to be incident on an objective lens off the axis thereof, and utilize the focusing effect (turning-back effect) of the objective lens. JP Patent Publication (Kokai) No. 2000-348658 A discloses a technique employing deflecting means in two stages for deflecting a charged particle beam in opposite directions in the focusing magnetic field of an objective lens. The technique corrects an off-axis chromatic aberration that is produced when the charged particle beam is tilted off the axis of the objective lens. JP Patent Publication (Kokai) No. 2001-15055 A discloses that deflecting means for passing the charged particle beam off the axis of an objective lens is disposed closer to an electron source than the objective lens. Chromatic aberrations (off-axis chromatic aberrations) produced off the axis of the objective lens are corrected by a Wiener filter disposed closer to the electron source than the objective lens, thus reducing the deterioration in resolution when the charged particle beam is inclined. Further, WO 01/33603 discloses a technique whereby a Wiener filter for generating a perpendicular electromagnetic field in arbitrarily chosen two-dimensional directions perpendicular to the optical axis is disposed on the optical axis closer to an electron source than an objective lens in order to correct for off-axis chromatic aberrations in arbitrary directions.
Patent Document 1: JP Utility Model Publication (Kokai) No. 55-48610 U (1980)
Patent Document 2: JP Patent Publication (Kokai) No. 2-33843 A (1990)
Patent Document 3: JP Patent Publication (Kokai) No. 2000-348658 A
Patent Document 4: JP Patent Publication (Kokai) No. 2001-15055 A
Patent Document 5: WO 01/33603
The method disclosed in JP Kokai 2000-348658 A, in which the charged particle beam is deflected in two stages in the magnetic field of the objective lens, has the problem that high resolution cannot be obtained when adapted to those systems in which the objective lens magnetic field is caused to leak towards the sample in order to obtain high resolution. Such systems are becoming increasingly common in recent years. The aforementioned problem is due to the fact that the distance between the magnetic poles of the objective lens and the sample has to be increased so that the two-stage deflection means can be disposed therebetween. This problem is addressed by JP Kokai 2000-348658, in which four magnetic poles for the objective lens are provided, and in which the combination of the magnetic poles can be switched depending on whether the purpose is high-resolution observation or the inclination of the charged-particle beam. In this method, however, as the number of magnetic poles is increased, axial misalignment among magnetic poles or other various problems (such as magnification error, axial misalignment, variations in scan conditions, etc.) that could be encountered when switching is performed must be solved before the method can be used in actual applications.
Generally, when a charged particle beam is tilted using the off-axis properties of an objective lens, not only an off-axis chromatic aberration but also a coma aberration are produced. While the off-axis chromatic aberration is dominant at low acceleration voltages, the coma aberration is more of a concern at relatively high acceleration voltages. Therefore, eliminating the coma aberration is important when the acceleration voltage is relatively high. Even at lower acceleration voltages, the coma aberration becomes large if the angle of inclination of the charged particle beam is increased, making it impossible to obtain high resolution even if the off-axis chromatic aberration is corrected. Thus, in order to obtain high-resolution images in cases where the charged particle beam is inclined at large angles, the off-axis chromatic aberration and the coma aberration must both be corrected at the same time. However, there is no consideration given in JP Kokai 2000-348658 to this issue, resulting in the problem of lowered resolution at high inclination angles.
In the technique disclosed in JP Kokai 2001-15055 A, the off-axis chromatic aberration produced as a charged particle beam is incident on an objective lens off the axis thereof is corrected using a Wiener filter. However, a Wiener filter is not capable of removing the coma aberration, resulting in a decrease in resolution in cases where the charged particle beam is inclined at large angles or when the charged particle beam is inclined at relatively high acceleration voltages.
Now referring to FIG. 2, the aberrations produced when the charged particle beam is inclined towards a sample using the turning-back effect of an objective lens will be described. A beam 4 is deflected by a beam inclination angle control coil 51 at the object point of an objective lens 7 such that the beam is incident on the objective lens 7 off its axis. As a result, the beam 4 is inclined towards a sample 10 due to the focusing effect of the objective lens 7. In this case, as the object point as seen from the objective lens 7 is not shifted, the field of view does not move even if the beam is inclined. By correcting the shift in the field of view when the beam is inclined, the beam in principle satisfies the deflection condition depicted in FIG. 2.
The aberration of the objective lens that is produced when the object point is on the optical axis consists of spherical aberration and on-axis chromatic aberration. The aberration (Δwi) produced by the objective lens can be expressed by a polynomial (1) shown below as a function of a trajectory inclination (w′i) on the sample. The trajectory curve is expressed as a differential equation of a trajectory function w (w=x+j·y, where j is the imaginary unit of a complex number) with respect to an optical axis z. A differential equation with respect to z will be herein indicated by a prime (“′”).Δwi=Csi·wi′·wi′· wi′+Cci·εi·wi′  (1)wherein
                              ɛ          i                =                              Δ            ⁢                                                  ⁢            V                                V            i                                              (        2        )            
Δwi: Aberration on the image surface
wi′; Inclination of trajectory towards the image surface
 wi′: Complex conjugate of w1′;
Cxi: Spherical aberration coefficient on the image surface
Cci: On-axis chromatic aberration coefficient on the image surface
ΔV: Variations in beam energy
Vi: Beam energy on the image surface
In the case where the beam is inclined by the turning-back effect of the objective lens, the trajectory curve (w′i) towards the sample can be expressed as the sum of a trajectory curve (w′t) corresponding to the beam inclination angle and a trajectory curve (w′f) associated with the beam opening angle as follows:wi′=wt′+wf′  (3) wi′= wt′+ wf′  (4)
FIG. 3 shows the condition of the beam in a trajectory curve coordinate system (w′i-coordinate system) on the sample. The horizontal axis in FIG. 3 indicates the gradient of the trajectory in the X-direction on the image surface, while the vertical axis indicates the gradient of the trajectory in the Y-direction. As the gradient of the trajectory is indicated by a differential equation with respect to z, the description has primes, as mentioned above. The circular figure (beam region) in FIG. 3 indicates the region of a set of trajectories of the primary beam having various gradients produced by the focusing effect of the lens. In FIG. 3, the center of the focusing trajectories of the beam corresponds to the beam gradient (w′t), so that the circular figure is displaced from the center of the coordinate system (xi′, yi′) in accordance with the beam gradient. The aberration when the beam is inclined can be obtained by substituting equations (3) and (4) into equation (1) as follows:Δwi=Csi·(wt′+wf′)·(wt′+wf′)·( wt′+ wf′)+Cci·εi·(wt′+wf′)  (5)
Equation (5) can be expanded to give equation (6), in which it will be seen that Seidel aberrations (spherical aberration, coma aberration, astigmatism, field curvature aberration, and distortion) and on-axis and off-axis chromatic aberrations are produced by the beam inclination (whereby the trajectory is displaced off axis) in the system in which originally there was only an on-axis aberration.Δwi=Csi·(wf′wf′ wf′+ wt′wf′wf′+2wt′wf′ wf′+2wt′ wt′wf′+wt′wt′ wt′+Cci·εi·(wt′+wf′)  (6)
These aberrations are listed below.Csi·wf′·wf′· wf′  (7): Spherical aberrationCsi·( wt′·wf′·wf′+2wt′·wf′· wf′)  (8): Coma aberrationCsi·(wt′·wt′· wf′+2wt′ wt′wf′)  (9): Astigmatism+field curvature aberrationCsi·wt′·wt′· wt′  (10): DistortionCci·εi·wt′  (11): Off-axis chromatic aberrationCci·εt·wf′  (12): On-axis chromatic distortion
Of the items in the above list, terms containing w′t are aberrations produced by the inclination of the beam, which are coma aberration, astigmatism, field curvature aberration, distortion, and off-axis chromatic aberration. However, only those aberrations that contain the beam focusing angle (w′f) and the function εi of the energy width ΔV (namely, coma aberration, astigmatism, field curvature aberration, and off-axis chromatic aberration) cause a deterioration in resolution when the beam is inclined.
Of those aberrations that cause deterioration in resolution when the beam is inclined, astigmatism can be easily corrected by means of a conventional astigmatism correction coil. The field curvature aberration, which is a focusing error due to beam inclination, can be eliminated by correcting the focusing condition (objective lens current). Further, the distortion, which is due to the displacement of the irradiated position caused by the beam inclination, can be eliminated by correcting the irradiated position in accordance with the beam inclination. Thus, the remaining aberrations to be considered are the off-axis chromatic aberration and the coma aberration, which both increase in proportion to the angle of beam inclination (w′t).